Don't need channel_id in activity body, and causes mass-assignment error in kandan

[JangoSteve]

Our Hubot was broken after we updated kandan. The kanan activities controller now scopes activities automatically to the channel id directly from the channel_id parameter. So the nested activity[:channel_id] parameter is no longer needed, and actually causes a mass-assignment error, causing hubot's post to fail.

[JangoSteve]

I should also mention that I submitted this update separately here:
kandanapp/hubot-kandan#12

But I had to make the change directly also in this project, because the hubot-kandan module, where this change was made, is required indirectly as a dependency of the direct hubot-kandan-heroku module dependency in this app, but that module no longer exists on Github, so there's no way to update that dependency to point to the updated hubot-kandan dependency.

If you check out the hubot-kandan-heroku npm module, you'll notice that its Github repo no longer exists.